Angular resolution in radio or sonar measurements, such as those used for radio astronomy, radar, or sonar, is traditionally achieved by summing up the signals coming to the antenna. If parabolic reflectors are used, the summation occurs by the reflector directing the signals to a single focal point, where they add up together at an antenna or waveguide horn (or transducer) placed at the focal point. As constructive interference of the signals only occurs from the direction coinciding with the axis of the antenna, a narrow beam pattern is generated, facilitating high angular resolution of the signals received.
Instead of parabolic reflectors, arrays of smaller antennas (linear, planar or conformal to some surface—regularly or irregularly situated) are also well known in the art. In this case the signal summation is typically done by simple analog electronic devices summing together the elementary contributions. This is often complemented with time delays of the individual signals corresponding to differences in signal distance from the target point or direction to the individual antennas. If the time delays can be made adjustable, the beam direction or target focus location can be changed by changing the time delays so that it fits particular needs. The time delays are often accomplished by using specially trimmed cable lengths and/or cable systems where different lengths can be switched to facilitate different time delays. Also, elaborate systems of cabling such as Butler matrixes are well known to accomplish several different summations of signal copies to occur simultaneously and thus to facilitate formation of several beams at the same time.
Instead of a narrow beam in certain direction(s), the goal may also be to form a wider beam than the array would typically produce and thus to cover a wider angular view for target searching and detection purposes, for example. These kinds of techniques have been more recently developed and are called antenna coding in the literature. Also here summation of individual signals with somehow chosen weighting coefficients and time delays is the way this is accomplished.
Many of the tasks previously done by analog systems can be replaced by digital signal processing. Beam forming (or antenna coding) is one of these applications. In this case signals from individual antennas are sampled and the necessary time-delayed and weighted signal sums are performed by digital hardware such as computers or FPGA (Field Programmable Gate Array) or ASIC (Application Specific Integrated Circuit) based digital systems.
FIG. 1 illustrates a principle of a digitally controlled system where reception takes place through an antenna array. A number of antennas, an example of which is antenna 101, are available to receive a signal. From each antenna the received signal is conducted to a delay or phasing unit 102, which is configured to produce one or more delayed copies of each signal. A summing unit 103 receives the delayed copies and produces one or more summed signals. In other words, together the units 102 and 103 accomplish the beam forming mentioned above. Either of them may also be configured to perform antenna-specific weighting before the summing is done.
The further processing unit 104 comprises all such functions that a conventional receiver with one antenna would have. The signal it receives from the summing unit 103 resembles one that would be received with a very large single antenna used in place of the antenna array. An additional unit 105 is shown to include control, storage, and user interface functions. Through this unit the user may program the antenna-specific delays in the delay or phasing unit 102 and affect the way in which the summing is done in the summing unit 103.
It is easy to see that a large number of antennas in the array of FIG. 1 means an extremely large amount of cabling. An antenna array may have tens of thousands of individual antennas or even more, distributed over an area with dimensions measured in kilometers. If the antennas constitute a roughly square matrix with N antennas on a side, the number of connections increases in proportion to N2. The larger the system grows, the more complicated become tasks like cable ditch digging and protecting the cabling against animals and other environmental risks. Also the physical and logical task of arranging the inputs from tens of thousands of lines to the central beam forming unit becomes formidable.
Reference US 2009/231197 discloses a phased antenna array arrangement for transmitting and receiving wireless signals with adjustable beam-forming capabilities. The structure of the receiver part is shown in FIG. 5 and the transmitter part in FIG. 7. The principal method is shown in the flow chart of FIG. 8. As it is evident through these figures and the corresponding description, the signal is received in an antenna element, the signal is demodulated into baseband samples, and the baseband samples are combined with baseband samples received from one of the other receive modules, and the combined baseband samples are communicated to a central processing unit.
Concerning FIG. 8 and its related description in paragraph [0084], after the receive time offsets and phase shifts, i.e. complex weights, have been communicated from the control processor to the receive modules, a first of the receive array elements receives a signal via the antenna element. Upon receiving the signal, a receive processor of the receive module demodulates the signal, to determine baseband samples. The demodulation process includes both the analogue and digital demodulation processes. Continuing into paragraph [0085], the receive module combines the baseband samples with baseband samples received from one of the other receive modules.
As it is thus evident from the description and Figures of this reference, the received signal requires to be demodulated into the baseband before the combination process takes place, in all embodiments of the reference. Correspondingly, the transmitter side (FIG. 7) reveals that the delaying, filtering and weighting steps are performed to the signal firmly locating in the baseband, that is, before the mixer.